Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 03 2024 08:26:00 FOUS30 KWBC 030825 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 AM EDT Sat Aug 3 2024 Day 1 Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN FLORIDA PENINSULA, FLORIDA KEYS, AND PORTIONS OF THE SOUTHEAST AND MID ATLANTIC... ....South Florida... Tropical Depression #4 over western Cuba will become better=20 organized over the eastern Gulf west of the FL Peninsula over the next 24 hours, with the system expected to reach Tropical Storm status by 00Z 8/4 per the latest advisory from the National Hurricane Center. The guidance (both the CAM and global QPFs) indicate the heaviest rain just off the coast in SW FL. However, given the better destabilization east of the center, even if just=20 a little, with tall/skinny CAPEs of 1000-2000 J/Kg, rainfall rates will be quite prolific given the ample deep-layer theta-e/moisture profiles (TPWs 2.4 to 2.6+ inches). Given the current and projected forward speed of the TD through Saturday, it appears as the Slight Risk (higher-end Slight) will suffice. Many locations across SW FL (including FMY) will see between 3-5" per the guidance, with=20 lesser areal-average totals to the east. However, given the rich=20 thermodynamic environment, localized totals of 6-8+ inches are=20 anticipated within the Slight Risk area. The latest (00Z) HREF in fact shows 60-80% probabilities of 24hr rainfall exceeding 5" over a portion of SW FL along the coast, with 30-35% probs of exceeding 8". ....Southeast and Mid Atlantic... Farther north across the Southeast and Mid Atlantic, ample low- level moisture transport/flux and anomalous PWs ahead of a slow- moving mid/upper level trough and surface front will allow for numerous showers and thunderstorms across much of the Eastern Seaboard up through the northern Mid-Atlantic Region and southern NY-southern New England. MLCAPEs of 1000-2000+ J/Kg during peak heating along with PWs of 1.75-2.0+ inches will make for hourly rainfall rates of 2+ inches underneath the strongest cells, with localized totals of 3-5" during the period per the CAMs and latest HREF probabilities. Urban areas will be at the highest risk for=20 flash flooding within the broad Slight Risk area. ....Southwest... Monsoonal convection over the Southwest (precipitable water values up to 1.75" near the International Border) continues to support a Marginal Risk area. Locally heavy rainfall would be most problematic in any area burn scars and dry washes/arroyos. ....Sierra Nevada into Oregon Cascades, Blue Mountains, and parts of the Northern Rockies to western portions of the Great Basin... Anomalous moisture (TPW anomalies topping 3-3.5 standard deviations above normal per the GEFS) will advect northward through the West on the backside of the upper ridge and ahead of the shortwave trough off the PAC NW coast. The guidance gets TPW values up to 1-1.25", which would be most anomalous in terrain. Given these moisture profiles, along with the diurnal heating (MLCAPEs peaking at least ~500 J/Km), spotty hourly or sub-hourly rainfall rates aoa 1" will make for localized runoff issues, especially over burn scares. ....Upper Midwest... The guidance is generally showing a modest signal for heavy rainfall in and near Minnesota this period. Precipitable water values rise about 1.25" with a warm advection pattern. This led to an introduction of a Marginal Risk here, as flash flood guidance values are modest after recent heavy rainfall. Hurley Day 2 Valid 12Z Sun Aug 04 2024 - 12Z Mon Aug 05 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE FLORIDA PENINSULA AND PARTS OF THE SOUTHEAST COASTAL PLAIN... ....Florida through the Southeast Coastal Plain.... TD #4 will eventually evolve into a defined tropical cyclone with=20 the latest NHC forecast developing the system into our next=20 tropical storm later this (Saturday) evening, carrying through the D2 time frame. The uncertainty of exact location of the low given=20 the timing and interaction with the approaching trough/front to the north has limited the capabilities for discerning specifics for=20 the Sunday into Monday period. There is some general consensus on=20 heavy rainfall potential within the Western FL coastal plain,=20 including areas like Sarasota up through Tampa and Clearwater, as the expected tropical storm lifts west of the FL Peninsula before expected to make landfall just east of the FL Big Bend. Many of the models keep the heaviest rain just offshore (closer to the center), while for now solid model consensus is somewhat lacking with the=20 QPF during this period (Sun-Sun night). For this reason, per collaboration with the FL offices, have maintained an enhanced Slight Risk in the Day 2 ERO, opting for now to hold off on a Moderate.=20 Meanwhile, some of the guidance continues to develop a Predecessor Rain Event (PRE) across the Southeast coast thanks to the timing=20 of the system's approach with interaction of the coastline. This=20 something to monitor in future updates as these phenomena are tough to gauge at range, but can be focused significant rain events=20 where they occur. Regardless if a PRE forms or not, the highly=20 anomalous tropical moisture flux and persistent heavy rain banding=20 around the storm circulation will cause locally enhanced flash=20 flood concerns across much of FL into the GA/SC coast. The SLGT=20 risk was maintained given the ensemble QPF continuity and expected=20 NHC track with the opportunity for future upgrades pending=20 increased confidence on the storms forward motion. ....Montana into the Northern Plains and Upper Mississippi Valley... Aforementioned axis of anomalous TPW out West during Day 1 will=20 ultimately get absorbed in the upper level jet streak and mid-=20 level vort lobe that will cross MT and the Dakotas on Day 2.=20 Dynamical forcing along with the thermodynamical profile will favor more widespread showers and thunderstorms, though the shear=20 profiles will certainly favor more progressive/forward-propagating=20 cell motions and thus make for more of an isolated flash flood=20 threat. Areas across northwest MT, including Glacier NP, would be=20 the most susceptible given the lower FFG (1" or less within 3 hours for most areas). An upgrade to Slight may be needed over this=20 region in later outlooks, especially if we start seeing a consensus of heavier QPF from the models. ....Southwest.. Monsoonal moisture advecting on the underside and backside of the mid-upper level ridge will once again generate scattered showers and thunderstorms during the afternoon into evening peak heating hours. MLCAPEs of 500-1000 J/Kg will favor pockets of 1"/hr rainfall rates, which will likely lead to localized areas of flash flooding, especially over burn scars and arroyos. Hurley Day 3 Valid 12Z Mon Aug 05 2024 - 12Z Tue Aug 06 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHEASTERN COASTAL PLAIN... ....Southeast... Per collaboration with the WFOs, have hoisted a Moderate Risk during Day 3 (Mon-Mon night) across NE FL through the eastern coastal plain of GA-SC. This based on the latest NHC track and speed of TD #4, along with the anticipated rainfall from the day prior (Sun-Sun night). Expect further refinements in the outlook area over the next several forecast cycles, as there are still=20 some location and timing differences with the models. UKMet and ECMWF in fact are on the lower edges of the guidance spread (UKMet farther west/inland with the QPF max), CMC much farther inland=20 with the heaviest rain (central-northern GA), while the 00Z GFS QPF appears to align best with the NHC forecast track of TD #4. With=20 the TC's forward speed expected to slow to 6-8 kts over SE GA by=20 Day 3, that would put TD #4 in the 20th percentile of storm forward speed per climatology. As a result, areas of 5-10", perhaps with=20 higher max totals, will be possible within the Moderate Risk area.=20 Again, later shifts will refine the outlook, while also assessing=20 the potential need for a High Risk. The slowing forward speed by=20 Day 3 would be a big concern as such would elevated the excessive=20 rainfall threat considerably.=20 Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4lzKA7UUNirUADl1744R62qoMUQMjnBZCj_sru_BW4r3= j6v-NYD4y2y6fp2CHr_T49JlWP1xr2TIExR5Y6Cs8Jr_gh0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4lzKA7UUNirUADl1744R62qoMUQMjnBZCj_sru_BW4r3= j6v-NYD4y2y6fp2CHr_T49JlWP1xr2TIExR5Y6CsWUD84Do$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4lzKA7UUNirUADl1744R62qoMUQMjnBZCj_sru_BW4r3= j6v-NYD4y2y6fp2CHr_T49JlWP1xr2TIExR5Y6CsZ7g7WS0$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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